Chemical modifications of aminoglycoside antibiotics have been investigated intensively to enhance antimicrobial activity and reduce side effects. The inventors have found that chemical modification of the 8'-hydroxy group of aprosamine which is prepared by hydrolysis of Apramycin (for example, described as Nebramycin factor II in Jap. Exam. Pat. Pub. No. 51-36358) enhances the antimicrobial effect. The invention was completed on this result.